PPP and tunnels

Many protocols can be used to tunnel data over IP networks. Some of them, like SSL, SSH, or L2TP create virtual network interfaces and give the impression a direct physical connections between the tunnel endpoints. On a Linux host for example, these interfaces would be called tun0.

As there are only two endpoints on a tunnel, the tunnel is a point-to-point connection and PPP is a natural choice as a data link layer protocol between the virtual network interfaces. PPP can assign IP addresses to these virtual interfaces, and these IP addresses can be used, for example, to route between the networks on both sides of the tunnel.IPsec in tunneling mode does not create virtual physical interfaces at the end of the tunnel, since the tunnel is handled directly by the TCP/IP stack. L2TP can be used to provide these interfaces, this technique is called L2TP/IPsec. In this case too, PPP provides IP addresses to the extremities of the tunnel.

November 24, 2011

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